2017A&A...598A..34S

We report on the discovery of coherent pulsations at a period of 2.9 ms from the X-ray transient MAXI J0911-655 in the globular cluster NGC 2808. We observed X-ray pulsations at a frequency of ∼339.97Hz in three different observations of the source performed with XMM-Newton and NuSTAR during the source outburst. This newly discovered accreting millisecond pulsar is part of an ultra-compact binary system characterised by an orbital period of 44.3-min and a projected semi-major axis of ∼17.6lt-ms. Based on the mass function, we estimate a minimum companion mass of 0.024M_☉, which assumes a neutron star mass of 1.4M_☉ and a maximum inclination angle of 75° (derived from the lack of eclipses and dips in the light-curve of the source). We find that the Roche-lobe of the companion star could either be filled by a hot (5x10⁶K) pure helium white dwarf with a 0.028M_☉ mass (implying i~=58°) or an old (>5Gyr) brown dwarf with metallicity abundances between solar/sub-solar and mass ranging in the interval 0.065 to 0.085 (16<i<21). During the outburst, the broad-band energy spectra are well described by a superposition of a weak black-body component (kT∼0.5keV) and a hard cut-off power-law with photon index Γ∼1.7 and cut-off at a temperature kT_e∼130keV. Up until the latest Swift-XRT observation performed on 19th July, 2016, the source had been observed in outburst for almost 150 days, which makes MAXI J0911-655 the second accreting millisecond X-ray pulsar with outburst duration longer than 100 days.